Photonic crystals are spatially periodic structures having useful electromagnetic wave properties, such as photonic bandgaps. In principle, the spatial periodicity of a photonic crystal can be in one, two, or three dimensions. There is especially high interest in developing technology of artificial photonic crystals that are useful in new and improved functional photonic devices, especially for the infrared and visible-light portions of the electromagnetic spectrum. Functional devices using photonic crystals, such as selective reflectors, filters, optical couplers, resonant cavities, delay lines, and waveguides have been proposed and/or fabricated.
Several methods for forming artificial photonic crystals are known. Multilayered dielectric films have been used to make one-dimensional photonic crystals along the dimension perpendicular to the films.
Three-dimensional photonic crystals have been formed by stacking and bonding wafers in which periodic structures have been micro-machined by etching. Such methods result in structures called “wood-pile” or “picket-fence” structures because the stacked elements have an appearance similar to stacked square timbers. Such methods require precise alignment of the micro-machined wafers to be bonded together, which becomes more difficult as the number of layers increases and as the dimensions of micro-machined features are reduced.
Some of the known methods for forming artificial photonic crystals work by modifying the refractive index periodically in a material originally having a uniform refractive index. For example, light-wave interference or holography has been used to create periodic variations of refractive index within photosensitive materials, such as photoresist, to make photonic crystals. Perhaps the simplest methods for forming a one- or two-dimensional photonic crystal are those methods that form a periodic or quasi-periodic array of holes in a uniform slab of material. A vacuum or material filling the holes has a different index of refraction from the base material of the slab. In the background art, such holes have been formed by micro-machining or by nanoscale lithography, such as electron-beam or ion-beam lithography. Conversely, such charged-particle beam lithography has also been used to selectively assist deposition of material to form spaced elements of the photonic crystal.
Some photonic crystals have been formed by self-assembly of very small particles provided in a colloidal suspension. A colloidal suspension is used (e.g., in a Langmuir-Blodgett type of process) to form a periodic array of nano-particles (e.g., nano-spheres). This structure can then be backfilled using atomic-layer chemical vapor deposition (ALCVD), for example. The colloidal structure can be removed, thus forming an inverse opal structure.
Another approach has been to use substantially smaller nano particles along with the larger nanospheres in a suspension. As the material is dried in a sedimentation process, pressed and then sintered, a periodic structure is formed. The nanospheres can be removed, resulting in an inverse opal structure. Thus, when the colloidal particles themselves have been removed to leave an “inverse” photonic crystal, the crystal lattice positions are occupied by voids in a matrix.
In another example of a colloidal process, nanocrystals have been assembled from a colloidal suspension, concentrated as close-packed clusters in pores in a template (the pores being larger than the nanocrystals) to form a quantum-dot solid. In some cases, the interstitial spaces between the colloidal particles have been filled with a second material of a different refractive index.
While all of these methods and others have been used successfully to make small quantities of photonic crystals, more efficient and lower-cost methods for mass-production fabrication of photonic crystals are needed.